The present invention relates to an apparatus for removing harmonic components of electric power generated in electric power generation equipment at the time of starting of or in rated operating conditions of a turbine-driven generator.
Gas turbine electric power generation equipment using a gas turbine, and combined cycle electric power generation equipment using a gas turbine and a steam turbine in combination, as a prime mover are conventionally known. As is well known, a gas turbine of an ordinary type is provided with a compressor directly connected to the gas turbine so that a mixture of air compressed by the compressor and fuel such as natural gas is combusted to obtain a combustion gas, and the combustion gas is supplied to the turbine to make the turbine rotate to thereby generate motive power, while the compressor is driven to rotate by the gas turbine to thereby carry out its operation continuously. Accordingly, in any case the compressor must be driven to rotate to obtain compressed air at the time of starting of the turbine.
In the case of gas turbine electric power generation equipment or the like, it is therefore necessary that torque be supplied from the outside to the gas turbine to maintain the rotating of the gas turbine when torque generated by the gas turbine is lower than the sum of torque (internal torque) required for driving the compressor and the torque (load torque) required for driving the generator.
As a result, in the case of gas turbine electric power generation equipment or in the case of combined cycle electric power generation equipment, a starter for driving the gas turbine to rotate is required until the torque generated by the gas turbine becomes higher than the sum of the internal torque and the load torque so that the gas turbine becomes able to increase its rotational speed on its own.
As a starter which is adapted to this case, conventionally, such an apparatus using an induction motor as shown in FIG. 6 has been used.
In the electric power generation equipment shown in FIG. 6, a generator 5 is driven to rotate by a gas turbine 6 to thereby generate electric power, the voltage of the generated electric power is converted into a predetermined value by a transformer 8, and the electric power with the predetermined voltage is supplied to an electric power system through a circuit-breaker 7.
In the electric power generation equipment, a rotation shaft extending from on side of the gas turbine 6 and a rotation shaft of the generator 5 are directly connected concentrically and coaxially with each other, while a rotation shaft extending from the other side of the gas turbine 6 is connected to an induction motor 20 through a torque converter 19. At the time of starting of the gas turbine 6, the driving power generated by the induction motor 20, rotated by electric power supplied from the electric power system, is converted into predetermined torque by the torque converter 19 so that the predetermined torque is transmitted to the gas turbine 6 to thereby start the gas turbine 6.
In recent years, large-capacity gas turbine electric power generation equipment or combined cycle electric power generation equipment using a gas turbine has been built to satisfy an increased demand for electric power. With the increase of the capacity of electric power generation equipment, it becomes necessary that a large-capacity motor and a large-capacity torque converter are used in the aforementioned starter. It is, however, particularly difficult to produce such a large-capacity torque converter in accordance with the increase of the capacity of electric power generation equipment without a remarkable increase in both the size and the cost of the torque converter.
Further, because the induction motor which has been used in the conventional starter generally requires a large starting current at the time of starting thereof, there is an increased risk of causing a large disturbance to the electric power system. Also in this respect, it is difficult to apply the conventional starter to the electric power generation equipment.
In addition, in the case where a plurality of gas turbines are used, it is necessary to provide a plurality of such conventional starters in one-to-one correspondence with the plurality of gas turbines. Also in this respect, increase in both the space requirement and the cost cannot be avoided.
Upon such circumstances, in recent years of attention has been focused on a starter called a "thyristor starting system".
The thyristor starting system is a starter in which a frequency converter supplied with an electric power from an electric power system is used, and variable-voltage variable-frequency electric power is supplied from the frequency converter to the aforementioned gas turbine generator to operate the generator as a motor, to thereby start the aforementioned gas turbine generator. Because thyristors are mainly used in the frequency converter, this starter system is called "a thyristor starting system".
According to the thyristor starting system, the frequency converter is necessary for starting the gas turbine generator, but it is unnecessary that the induction motor, the torque converter, etc. be used separately. It is further unnecessary to increase the size of the generator in order to perform starting because torque necessary for starting the generator never exceeds torque of the generator in the rated rotating conditions. Accordingly, because there is no risk of increase in both space requirement and the cost, the spotlight of attention has been focused on the thyristor starting system in recent years.
Apparatus of this type is disclosed in the following patent publications:
(1) U.S. Pat. No. 3,591,844 PA1 (2) JP-A-3-70825 PA1 (3) JP-A-4-29600 PA1 i=1, 2, 3, . . . PA1 f.sub.0 =frequency of the fundamental wave PA1 A.sub.0 =amplitude of the fundamental wave
In the conventional technique of the aforementioned thyristor starting system, however, there is no consideration given to the fact that the starting of the generator is accompanied by an increase in the temperature of the rotor surface of the generator. Accordingly, the conventional technique has a problem in that there is a risk of damaging the generator.
That is, because a synchronous generator is mainly used as the gas turbine generator, the rotational speed of the generator depends on the frequency of the AC power supplied to the armature winding when the generator is operated as a motor.
In the conventional starter using the aforementioned thyristor starting system, therefore, variable-voltage variable-frequency electric power is obtained by using a frequency converter so that variable speed control necessary for starting the generator can be obtained. As is well known, such a frequency converter includes by an electric power rectification portion for converting AC power into DC power, and an electric power inversion portion for converting DC power into AC power.
At the time of starting of the gas turbine generator, the electric power inversion portion of the frequency converter supplies AC power to the armature of the generator. As is well known, the electric power inversion portion converts DC power into AC power by the switching operation of semiconductor devices. Accordingly, the current i.sub.L outputted from the electric power inversion portion of the frequency converter is a rectangular current as shown in the waveform (a) of FIG. 3, that is, an armature current i.sub.L.
Accordingly, the armature current i.sub.L contains harmonic current components of frequency f.sub.i and amplitude A.sub.i follows: EQU f.sub.i =(6i.+-.1)f.sub.0 EQU A.sub.i =A.sub.0 /(6i.+-.1)
wherein
As described above, a thyristor starting system, an armature current i.sub.L containing harmonic current components flows in the armature of the generator at the time of starting of the generator. As a result, an eddy current is induced in the surface of the rotor of the generator, so that heating occurs to cause the rotor temperature to increase.
Referring to FIG. 7, there is shown a synchronous generator rotor 13 used in a gas turbine generator or the like, for explaining the state of the eddy current induced upon starting. In the rotor 13, currents flowing in wedges 15, which are provided in the outer circumferential side of a winding so that the winding is not separated from slots by centrifugal force, go to teeth 14 between winding slots at junctions 15a of the wedges 15. Then, these eddy currents go to a retaining ring 18 and a damper ring 16 at an end of the rotor 13 and flow around the circumference of the rotor.
In a pole of the rotor, currents in the vicinity of cross slots 17, provided for maintaining a balance between the weight and the strength in the direction of the circumference of the rotor, are concentrated in respective ends of the cross slots 17.
Power is lost as a result of these eddy currents, so that the temperature of the surface of the rotor 13 rises.
On the other hand, in addition to this, sufficient cooling cannot be achieved in a low rotational speed condition at the time of starting of the generator because the cooling characteristic of the rotor depends on the rotational speed thereof. As a result, in the conventional technique, a risk of temperature increase in the generator occurs at the time of starting of the generator.
As a conventional technique for preventing such temperature increase in the generator, a gas turbine starting method is disclosed in JP-A-4-54227. According to this conventional technique, a variable-frequency electric source for operating the generator as a synchronous motor is provided so that electric power containing smaller harmonic components is supplied at the time of starting of the generator.
Although the above description has been made with reference to the starting-up condition that harmonic components are contained in an electric power system not only in the starting-up condition but also in the rated operating condition of the generator, so that the harmonic components cause a high temperature in the generator to thereby lower the quality of the commercial electric power.